Using two decades of data from the Hubble Space Telescope, scientists have discovered seven unusual stars in Omega Centauri that potentially point to the existence of an intermediate-mass black hole closest to Earth and change theories about the environment of black holes.

An international team of astronomers has used more than 500 NASA/ESA Hubble Space Telescope images spanning two decades to reveal seven fast-moving stars in the inner region of Omega Centauri, the largest and brightest globular cluster in the sky. These stars provide compelling new evidence for the existence of an intermediate-mass black hole.

A black hole was discovered in Omega Centauri

Intermediate-mass black holes (IMBs) are the long-sought “missing link” in the evolution of black holes. To date, only a few candidates for the role of IMBH have been found. Most known black holes are either extremely massive, such as the supermassive black holes found at the nuclei of large galaxies, or relatively lightweight, with masses less than 100 times that of the Sun. Black holes are one of the most extreme environments known to man, and as such are a testing ground for the laws of physics and our understanding of how the universe works. If IMBHs exist, how common are they? Does a supermassive black hole grow from a black hole? How do CDDs themselves form? Are dense star clusters their preferred home?

Observations from the southern sky

Omega Centauri is visible to the naked eye from Earth and is one of the most popular celestial objects for Southern Hemisphere astronomers. Although the cluster is 17,700 light-years away, just above the plane of the Milky Way, it appears nearly as large as the full Moon when viewed from the dark countryside. Omega Centauri’s precise classification has changed over time as our ability to study it has improved. It was first listed as a single star in Ptolemy’s catalogue almost two thousand years ago. Edmond Halley described it as a nebula in 1677, and in the 1830s, English astronomer John Herschel was the first to recognize it as a globular cluster.

Globular clusters typically consist of about a million old stars tightly bound together by gravity and are found in both the outskirts and the middle of many galaxies, including our own. Omega Centauri has several characteristics that set it apart from other globular clusters: It rotates faster than a normal globular cluster and is very flat in shape. Omega Centauri is also about 10 times larger than other large globular clusters, making it almost as big as a small galaxy.

Detection of star movements in Omega Centauri

Omega Centauri consists of about 10 million gravitationally bound stars. An international team has studied more than 500 Hubble images of the cluster, measuring the velocities of 1.4 million stars to create a massive catalogue of their motions. Most of these observations were intended to calibrate Hubble’s instruments rather than for scientific use, but they have proven to be an ideal database for the team’s research efforts. The comprehensive catalogue, the largest catalogue of the motions of any star cluster to date, will be made public (more information here ).

“We discovered seven stars that shouldn’t be there,” said Maximilian Heberle of the Max Planck Institute for Astronomy in Germany, who led the study. “They are moving so fast that they should leave the cluster and never come back. The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center. The only object that could be that big is a black hole with a mass of at least 8,200 times that of the Sun.”

Evidence of an intermediate-mass black hole

Various studies have demonstrated the existence of an IMBH in Omega Centauri. ^[1] However, other studies have suggested that the mass could originate from a central cluster of stellar-mass black holes, and suggested that the lack of stars moving faster than the required escape velocity makes IMBHs less likely in comparison.

“This discovery is the most direct evidence yet for the existence of an IMBH in Omega Centauri,” added team leader Nadine Neumeier, also from the Max Planck Institute for Astronomy, who initiated the research with Anil Seth from the University of Utah in the United States. “This is exciting because very few other black holes with similar masses are known. The black hole in Omega Centauri may be the best example of an IMBH in our cosmic neighborhood.”

If confirmed at 17,700 light-years away, the candidate black hole would be closer to Earth than the 4.3-million-solar-mass black hole at the center of the Milky Way, which is 26,000 light-years away. It would also be the only known case of multiple stars closely associated with a supermassive black hole outside the center of the galaxy.

Future research and technological contributions

The team now hopes to characterize the black hole. Its exact mass and location are not known, although it is believed to measure at least 8,200 solar masses. The team also plans to study the orbits of fast-moving stars, which would require additional measurements of the relevant line-of-sight velocities. The team has been given time to use the James Webb Space Telescope to do just that, and has other offers pending to use other observatories.

Omega Centauri was also recently part of new data from ESA’s Gaia mission, which includes more than 500,000 stars. “Even after 30 years, the Hubble Space Telescope, together with its imaging instruments, is still one of the best tools for high-precision astrometry in crowded star fields, regions where Hubble can provide additional precision through observations from the ESA Gaia mission,” said team member Mattia Libralato, who was at the time of the study from the National Institute of Astrophysics (INAF) in Italy and formerly of AURA for the European Space Agency. “Our results demonstrate Hubble’s high resolution and sensitivity, which are providing us with exciting new scientific insights and will give new impetus to the topic of IMBHs in globular clusters.”